Despite the size advantage, UFC color commentator Joe Rogan has warned that Floyd Mayweather could suffer the first defeat of his career next month - when he heads to Japan to take on kickboxing superstar Tenshin Nasukawa on December 31.

On Monday, at a press conference in Japan, Mayweather shocked everyone by announcing that he finalized a deal with Rizin Fighting Federation for a fight with Nasukawa.

Nasukawa, 20, is widely viewed as a rising star in the country - and many regarded him as one of the best Japanese combat sports prospects in years.

The biggest mystery at the moment are the rules.

No rules were announced for the fight - and both sides hinted at the idea of a contest with modified rules.

As BoxingScene.com reported earlier today, the contest will likely be an exhibition fought under traditional boxing rules.

Nasukawa, who is 27-0 in kickboxing and 4-0 in MMA.

Mayweather is 50-0 in boxing and has never fought outside of the United States.

"I have fought in the US for all 50 of my fights," he said. "As a professional I haven't had a chance to go outside the US to display my skills and to display my talent for the world."

Mayweather has been out of the ring since scoring a ten round stoppage win over UFC superstar Conor McGregor in a crossover fight in August 2017.

Rogan, like many other combat sports insiders, has a lot of respect for Nasukawa.

Although Nasukawa fights as a featherweight, Rogan gives him a huge edge if kicks are allowed in any manner within the finalized rules.

"I don’t know what kind of ‘special rules’ they’re supposed to be fighting under, but I just like the fact that it’s going to happen and that people get to Google Tenshin’s name and see some of his fights. He’s a true striking genius. I doubt Floyd is going to agree to any rules that allow Tenshin to kick, but if he does, it could be a terrible night for him. This kid is the truth," Rogan said.